543 On the cr\j]iaU'ine Forms of Corundum. 



angles at the fummit * will meafure 84° 31' ; and that taken at the reunion of thebafis will 

 be 95° 29'. 



"We can fplit this parallelopiped only in a direftion parallel to its faces ; it will ftill con- 

 fequendy preferve the fame form, which is that of the nucleus of this fubftance, and its 

 primitive form. It is, therefore, hy a modification of the rhomboidal parallelopiped 

 (fig. 2.) that nature has formed the regular hexaedral prifm (fig. i.) which this fubftance 

 prefents. 



For if we conceive that in any period whatever of the increafe of the rhomboidal 

 parallelopiped, a feries of laminae, or cryftalline plates, has been depofited on all the fides of 

 the parallelopiped ; and that thefe laminse have all undergone a progreffive decreafe of one 

 row of cryftalline molecules, at the acute angle which tends to form the fummit ; and alfo 

 along the fides of the oppofite acute angle (fig. 3. and 4.); there will neceflarily refult from 

 the continuation of this fuperpofition, to a certain period, an hexaedral prifm, terminated 

 by two tricdral pyramids, placed in a contrary direftion ; and thgir planeS)^ or faces, which 

 form a folid angle, of 147° 26', with the fides of the prifm, will be either pentagonal 

 (fig. 3.) or triangular (fig. 4.). They will alfo have, in place of a fummit^ an equilateral 

 triangular plane, fometimes greater and fometimes fmaller. 



If the fuperpofition continues, the equilateral triangular plane, on the^mmlt, will be- 

 come nonagonal, and there will remain no other traces of the primitive planes of the 

 rhomboidal parallelopiped than fmall ifofceles triangular planes (fig. 5.) : if the fuperpofitioa 

 ftill continues, until the laft cryftalline lamina is reduced to a fingle molecule, or point, 

 no appearance of the rhomboidal parallelopiped will then remain ; and the cryftal refulting 

 from this operation of nature will be a regular hexaedral prifm (fig. i .). 



In the fame manner, viz. by a decreafe on the lower edges of the laminje, the primitive 

 rhomboidal parallelopiped of calcareous fpar pafles to a regular hexaedral prifm of that fub- 

 ftance, though more frequently it does fo by a decreafe on the lower angles of the laminae. 

 When the lamina; of the corundum cryftal have, during their fuperpofition on the planes 

 of the primitive rhomboidal parallelopiped, experienced a progreflive decreafe at one of their 

 acute angles, and along the fides of the other, at the fame time, and in the fame proportion, 

 it is eafy to conceive that the height of the hexaedral prifm muft be the fame as that of the 

 rhomboidal parallelopiped, upon which it has been formed. The height B C (fig. i.) 

 muft therefore bear the fame proportion to the line A B, drawn through the middle of 

 the two oppofite fides of the planes, on the extremities, as the whole height E F of the 

 rhomboidal parallelopiped (fig. 2.) bears to tlie fmall diagonal G H, from one of the rhombs, 

 that is, nearly as 6, 45 : 5. 



But although this exaft proportion appears in a very great number of corundum cryf- 

 t^ls, yet we meet with fome, whofe lengths are more or lefs confiderable \ and this is owing 



• For greater dearnefs, this rhomboidal parallelepiped may be confidcred as being formed bythejunflion 

 •f two triedral pyramids, bafe to bafe ; and the two folid angles (each of which is formed by the re- 

 union of three of the acute angles on the pl.ncs of the rhomb) will then be confidercd as the fiitnmitsof tbefe 

 fyraisidi. 



to 



